Add example of conservatively rejected recursion

testsuite/tests/typing-layouts-unboxed-records/recursive.ml

@@ -247,6 +247,19 @@ Error: Unboxed record element types must have a representable layout.
 (***************************************)
 (* Singleton recursive unboxed records *)
 
+(* CR layouts v7.2: allow the below. *)
+type 'a safe = #{ a : 'a }
+type x = int safe safe
+[%%expect{|
+type 'a safe = #{ a : 'a; }
+Line 2, characters 0-22:
+2 | type x = int safe safe
+    ^^^^^^^^^^^^^^^^^^^^^^
+Error: The definition of "x" is recursive without boxing:
+         "x" = "int safe safe",
+         "int safe safe" contains "int safe"
+|}]
+
 (* We could allow these, as although they have unguarded recursion,
    they are finite size (thanks to the fact that we represent single-field
    records as the layout of the field rather than as a singleton product).

typing/typedecl.ml

@@ -2077,6 +2077,11 @@ let check_well_founded_decl  ~abs_env env loc path decl to_check =
    types like [type t = #{ t : t }]. The current implementation gives more
    informative error traces, and leaves the flexiblity to switch to a less
    restrictive check in the future.
+
+   CR layouts v7.2: accept safe types that expand the same path multiple times,
+   e.g. [type 'a t = #{ a : 'a } and x = int t t],
+   either by using layouts variables or the algorithm from
+   "Unboxed data constructors - or, how cpp decides a halting problem."
 *)
 let check_unboxed_recursion ~abs_env env loc path0 ty0 to_check =
   let check_visited parents trace ty =